1. Field of the Invention
The present invention relates to a method of operating a burner of a heat generator and in particular, to a burner including a swirl generator for providing a combustion-air flow, a mechanism for injecting at least one fuel into the combustion-air flow, and a mixing section having a number of transition passages for passing a flow formed in the swirl generator into a mixing tube, wherein a combustion-air/fuel mixture is obtained. It also relates to a burner for carrying out this method.
2. Discussion of Background
The low-pollution burners having a lean premix which are used at present for the operation of heat generators, for example combustion chambers of gas turbines, are stabilized aerodynamically by recirculation zones, for example vortex breakdown. This stabilization is based on the return transport of hot products of combustion, which serve as an ignition source for the lean fuel/air mixture. During such stabilization, the temperature of the recirculated products of combustion also decreases when the flame temperature is low, and thus the thermal energy transported into the reduction zone is no longer sufficient for activating the reaction. The consequence thereof is that the flame is extinguished.
During the development of premixed burners operated with a liquid fuel, it has been found that the extinction limit of the flame is reached only at substantially lower temperatures. Since the flame velocity of a liquid fuel, for example fuel oil, is lower than that of a gaseous fuel, for example natural gas, this effect can only be attributed to the lower activation energy in the case of long-chain hydrocarbons. In the case of liquid fuels, this results in self-ignition delay times which are substantially shorter than that of a gaseous fuel.
EP-0 620 362 A1 has disclosed a method in which the shorter self-ignition delay time is utilized. This method concerns the operation of a combustion chamber which is designed for self-ignition and in which, in order to ensure reliable self-ignition of the gaseous fuel injected into the combustion chamber, when the temperature drops below a certain level of the hot gases introduced there, action is taken by means of a small quantity of another fuel having a shorter self-ignition delay time. However, this action is taken in such a way as to be isolated from a defined premix section of a burner, so that the auxiliary fuel which is introduced can act here as an ignition fuse, so to speak. The risk of any kind of backflash of the flame need not be feared here, since there is no premix section having an intensely swirled flow.
In burners of the newer generation, however, as have been disclosed by EP-0 321 809 B1, EP-0 780 629 A2, the concern is to create recirculation-stabilized zones in order to extend the operating range having a lean premix flame. Since aerodynamic stabilization is effected here by an intensely swirled flow, the indiscriminate introduction of a fuel having shorter self-ignition delay times for improving the stability with respect to the extinction limit of combustion with a fuel having a poor ignition quality must not lead to the risk of a backflash of the flame being increased.